Control Transformer Fusing

[lefeverj]

I thought I had an easy question but there seems to be a lot of variations on this site as to the proper way to fuse a control transformer. I have a 100VA transformer in which the primary side is 480vac and the secondary is 120vac. I believe i have a read in a couple of different places to fuse the primary side at 250% of the transformer rating. Is this correct? The secondary is fused at the rated amp load up to the VA rating of the transformer at 125%, is this correct?

 

[jraef]

Depends on several factors, that's why it seems confusing. The rules are different based on the size of the transformer and whether or not you have fusing on the secondary (because there are applications where you don't need it).

In YOUR application, you DO have secondary fusing and the secondary current is over 9A, so the primary CAN BE up to 250% of the primary current, rounded up. Most fuse mfrs will recommend 125% as the "optimal" protection, but keep in mind what they do for a living; sell fuses...

 

[kamenges]

The NEC percentages for transformers with primary and secondary protection always confused me.

The text for a secondary current less than 9A is:
250% primary current

The text for a secondary current of 9A or more is:
Not more than 250%

What distinguishes between those two values? The 9A or more case is pretty clear. Then 9A or less case is a little cloudy. I usually fuse quite a bit less than the 250% allowed so I don't have to look that closely. But it is worded a bit oddly.

Your 100VA transformer will have a secondary current less than 9A so at least that one is clear. The fuse needs to be no larger than 167% of the secondary rated current.

Keith

 

[abhirupbera]

The NEC percentages for transformers with primary and secondary protection always confused me.

The text for a secondary current less than 9A is:
250% primary current

The text for a secondary current of 9A or more is:
Not more than 250%

What distinguishes between those two values? The 9A or more case is pretty clear. Then 9A or less case is a little cloudy. I usually fuse quite a bit less than the 250% allowed so I don't have to look that closely. But it is worded a bit oddly.

Your 100VA transformer will have a secondary current less than 9A so at least that one is clear. The fuse needs to be no larger than 167% of the secondary rated current.

Keith

@ Kamenges How did you calculate secondary current would be <9Amp??
It would be <0.9 Amp, because its 100VA,not 1000VA So the secondary Fuse should be of 1Amp and primary fusing should be of 250mA for sure. Thank you!

 

[abhirupbera]

Depends on several factors, that's why it seems confusing. The rules are different based on the size of the transformer and whether or not you have fusing on the secondary (because there are applications where you don't need it).

In YOUR application, you DO have secondary fusing and the secondary current is over 9A, so the primary CAN BE up to 250% of the primary current, rounded up. Most fuse mfrs will recommend 125% as the "optimal" protection, but keep in mind what they do for a living; sell fuses...

@ jraef How did you calculate secondary current would be <9Amp??
It would be <0.9 Amp, because its 100VA,not 1000VA So the secondary Fuse should be of 1Amp and primary fusing should be of 250mA for sure. Thank you!